We are studying the regulation of expression of the immunglobulin gene family by attempting to answer two questions: (1) why do only cells of the B-lymphoid lineage synthesize immunoglobulins, (2) how do these cells transcribe only one or a few immunoglobulin genes, while leaving hundreds of other, similar immunoglobulin genes inactive? Our approach to these questions is to insert a cloned, rearranged kappa light chain gene into a plasmid in various configurations, to transfect the plasmid into various types of cells, and to determine whether the transfected gene is transcribed. We have shown that the complete kappa gene is transcribed after transfection into antibody-producing myeloma cells but not in non-lymphoid 3T3 or L cells. Hence, the different cell types are able to appropriately regulate the kappa gene even when not in its usual chromosomal environment. By deleting different parts of the cloned gene, we have shown that certain sequence elements actually downstream of the promoter are necessary for its transcription in myeloma cells. Most recently, we have localized the important downstream elements to a 200 base pair region of DNA. We are currently transfecting the kappa gene into a variety of lymphoid cell types (T and B) to study its developmental regulation.